A variety of sensors can be utilized to detect conditions, such as pressure and temperature. The ability to detect pressure and/or temperature is an advantage to any device exposed to variable pressure conditions, which can be severely affected by these conditions. An example of such a device is a catheter or a cartridge for hemodialysis machine, which of course, can experience variations in both temperature and pressure. Many different techniques have been proposed for sensing the pressure and/or temperature in catheters and cartridges, and for delivering this information to an operator so that he or she is aware of pressure and temperature conditions associated with a catheter or a cartridge and any fluid, such as blood flowing therein.
One type of sensor that has found wide use in pressure and temperature sensing applications is the Surface Acoustic Wave (SAW) sensor, which can be composed of a sense element on a base and pressure transducer sensor diaphragm that is part of the cover. For a SAW sensor to function properly, the sensor diaphragm should generally be located in intimate contact with the sense element at all pressure levels and temperatures.
One of the problems with current SAW sensor designs, particularly those designs adapted to lower pressure range applications, is the inability of conventional SAW sensing systems to meet the demand in low pressure applications. (e.g., 0 to 500 mmHg), while doing so in an efficient and low cost manner. Such systems are inherently expensive, awkward, and often are not reliable in accurately sensing air pressure and temperature. There is a continuing need to lower the cost of sensor designs utilized in pressure and/or temperature sensing applications, particularly wireless pressure sensors.
A number of conventional passive wireless technologies have been utilized in commercial and industrial applications. For example, an LC resonator (tank) sensor has the potential of being utilized as a low cost sensor with a low interrogation cost, but provides a relatively poor sensor performance. An LC tank with regulated power, an associated ASIC and so forth has also been implemented, such a device may be good for multiple sensing applications, but such a configuration presents a high sensor cost and a large area coil.
PVDF acoustic wave sensors are also known and are based on low cost materials such as polymer along with flexible components thereof, but PVDF acoustic wave sensors are typically both piezo-electric and pyro-electric (i.e., highly temperature dependent), and cannot be exposed to temperatures greater than 120° C. Additionally the dimensions for a PVDF sensor are frequency dependent, such that the higher frequencies increase the associated costs to the sensor. Another example of a conventional sensor is the magneto elastic sensor, which is based on a low cost material, but such a sensor has difficulties in overcoming ambient RF effects, while also possessing long term stability issues associated with permanent magnetic materials, temperature variation and hysteresis problems.
A further example of a passive sensor is a quartz SAW sensor, which is generally accurate but results in a high sensor cost (i.e., material and etching) and trimming thereof lowers the Q value, and hence the performance of the device. Additionally the interrogation electronics (IE) for such a device can be expensive. A PZT SAW sensing device presents an alternative. The PZT material possesses a high coupling coefficient, which makes it possible working in low pressure range without an etched diaphragm. But it is difficult to control PZT chemistry (e.g., grain size, orientation, residual stress, control of nucleation at the at the electrode/PZT interface, Zr/Ti ratio control). PZT SAW sensor devices are also plagued with issues related to electrode materials migration, another drawback to the use of such a device.
A further example of a conventional sensor is the LiNbO3 SAW sensor, which also possesses high coupling coefficient/sensitivity. Such a sensor can be utilized in low pressure measurement without using a costly etching step. The LiNbO3 SAW sensor possesses a lower cost structure than the Quartz SAW sensor, but remains too high for cost-effective applications, particularly when considering its associated sensor packaging and interrogation electronics (IE) costs. Optical sensors represent additional types of conventional sensing applications and utilize optical fibers positioned in front of a flexible reflecting diaphragm. The intensity of the reflected light is related to the pressure induced deflections of the diaphragm. The relative position needs reference sensor, and therefore the alignment is difficult to control in sensing applications. Another drawback to optical sensors is that such devices are susceptible to environmental interference.
To lower the cost and raise efficiency, few components, less expensive materials and fewer manufacturing-processing steps are necessary. In order to achieve these goals, it is believed that a disposable LC tank pressure sensor should be implemented, along with reusable wireless interrogations. To date, such components have not been adequately achieved.
One area where the ability to detect pressure and/or temperature is critically important is in medical applications. Pressure in and out during a dialysis routine, for example, can be measured utilizing several techniques. Perhaps the most common methodology for such pressure measurement involves the use of a pressure sensor, which may be molded inside one wall of the conduit to the fluid pressure within the conduit. Inside the gauge, a needle is deflected over a scale in proportion to the pressure within the conduit. In some instances, the standard pressure gauge may be replaced with a transducer which converts pressure into an electrical signal which is then monitored. One important medical application for a pressure sensor involves detecting a patient's blood pressure, and/or intracranial pressure.
One typical method of monitoring blood pressure is to measure the fluid pressure within an intravenous tube which is hydraulically coupled to the patient's vein. A catheter is inserted into the patient's vein and a plastic tube or conduit coupled to the catheter. A saline solution can be drip-fed through the plastic tubing or conduit to maintain a pressure balance against the pressure within the patient's vein. The saline fluid acts as a hydraulic fluid to cause the pressure within the plastic tubing to correspond to the pressure within the patient's vein. Hence, by measuring the fluid pressure within the tubing, the patient's blood pressure will be known.
Conventional pressure sensors are expensive to implement in medical applications, rendering their wide-spread use limited, particularly in medical applications. It is therefore believed that a solution to such problems involves a disposable low cost sensor packaging system, particularly one which is suited to medical applications.